1. Field of the Invention
The present invention relates to a superconducting junction using an oxide superconducting material.
2. Description of the Prior Art
A superconducting tunnel junction is commonly known as a Josephson junction and is expected to find considerable application as a high speed device. Conventionally, a junction comprising a pair of superconductors and a thin(100 to 10,000 .ANG.) insulating film, a thin conductor film, a thin good conductor film, or a thin semiconductor film or the like interposed between the superconductors is known as a Josephson junction. Considerable research is being conducted into the utilization of a Josephson junction as an ultrahigh speed computer element., see H. Nakagawa et al.: 1986 Applied Superconductivity Conference. In particular, in the case where the insulating film is used, it is known as an SIS junction; when a conductor film or a semiconductor film is used, as an SNS junction. The characteristics of these two junctions differ slightly.
Conventionally, a metallic material with a low critical temperature has been used as a superconductor, with liquid helium used as a cooling medium. In recent years, oxide superconductors with critical temperatures exceeding the temperature of liquid nitrogen, or even exceeding an absolute temperature of 90 degrees, have been discovered could see H. Maeda et al.: Japanese Journal of Applied Physics Vol. 27, No. 2, February, 1988 pp. L209-L210. These oxide superconductors can be represented by the chemical formula Bi.sub.2 Sr.sub.2 Ca.sub.n-1 Cu.sub.n O.sub.2n+4 (where n=1,2,3,. . .). Trials are being conducted to fabricate tunnel junctions using these oxide superconductors.
Because the coherent lengths of these oxide superconductors are very small, ranging from 1 angstrom to dozens angstrom, it is very difficult to form the SIS type of tunnel junction using the oxide superconductors. The reason for this is that, in order to form a Josephson junction with good characteristics, the tunnel barrier must be an ultrathin film with a thickness not more than 100 .ANG.. In particular, this type of tunnel junction, which comprises a thin lower superconducting film, a thin tunnel barrier film on the thin lower superconducting film, and a thin upper superconducting film on the thin tunnel barrier film, is fabricated in the form of these three thin films and since the c-axes in the thin films are parallel to the thickness direction of the thin films, the tunnel barrier film must have a thickness up to several angstrom units which is substantially the same as the coherent lengths of the superconducting layers in the c-axis directions. It is impossible to form such a thin film by the current technology.
Owing to the progress occurring in a thin film formation technology, it has become possible to grow the thin film of an oxide superconductor epitaxially on a suitable substrate, see N. Terada et al.: Japanese Journal of Applied Physics Vol. 27 No. 4, April 1988, pp. L639-L642. However, it is impossible to further form a tunnel barrier film of substantially less than 100 .ANG. on the thin film and then properly form an oxide superconducting film in good crystallization on this tunnel barrier film.
The reason for this is as follows. The crystal structure of the magnesium oxide film, aluminum oxide film and the like used in a tunnel junction together with a pair of conventional metallic superconductor films differs from the crystal structure of Bi.sub.2 Sr.sub.2 Ca.sub.n-1 Cu.sub.n O.sub.2n+4 and therefore the formations of these thin films in good crystallization for a junction are impossible, so that it has not been possible to form a good Josephson junction.
In the SNS junction, the thickness of the tunnel barrier layer depends on the mean free path of the carrier in the material making up the tunnel barrier layer. Taking gold as an example, the thickness of the tunnel barrier layer is about 100 .ANG. at 77 K so that there are no problems from the aspect of fabrication. However, in order to obtain good superconductivity, the superconducting layer on the tunnel barrier layer must have a single crystalline structure identical to the superconducting layer under the tunnel barrier layer. Accordingly, in the process for successively laminating the superconducting layer, the tunnel barrier layer, and the superconducting layer, these layers must be formed by heteroepitaxial growth. This means that the use of gold, silicon, etc. having a crystal structure completely different from that of the oxide superconducting layers was suitable for the conventional SNS junction but is not suitable for the SNS junction using an oxide superconductor.